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There are numerous relevant details here, so read carefully.

I'm helping a video rental store switch to a new POS (cash register) software. Their previous software came pre-installed on 3 Windows 7 PCs which were purchased from the software publisher. Computer #1 contains SQL Server which serves the POS software on all 3 computers. This was the old set-up. (This may or may not be relevant.)

The new software will be using a remote, offsite SQL Server. I've installed the new POS software on all 3 computers; they will simply point to a different SQL server than the previous software did.

Unfortunately, I cannot get the new software to connect to the new SQL Server from all 3 of the computers. On computer 1, which was previously the server for the old software, the new software fails to find the new server. Instantly, with no lag time whatsoever, the software simply reports that the target server could not be found. However, on that same computer, I have no trouble connecting to the new SQL Server using SQL Server Management Studio. Ping also has no trouble reaching the remote server. It's only the POS software that can't find/connect to the new SQL server.

On computers #2 and #3, the new POS software CAN connect successfully to my new SQL server. This tells me the POS software itself is probably not at fault. There's something else on Computer #1 stopping the connection from succeeding.

I disabled the McAfee firewall on computer #1 with no effect. The POS software still failed to connect. I checked the router settings for possible firewall issues there. Nothing. I disconnected the network cable from Computer #1 and connected it to my laptop where I was able to connect with no problem.

It's just this 1 application on this one computer that can't reach the remote SQL Server. Well actually, there is a companion app which has the same problem, but again, only on Computer #1. Since the companion app has the same problem I can't blame a corrupt installation. The companion app does not share installed components with the main app as it is a small standalone executable.

I've double-triple-quadruple-checked the server address, login, password, etc. The software does not appear to use ODBC.

What could the problem possibly be? Besides the McAfee firewall software, is there somewhere else in the system that could be blocking this app from connecting to the remote SQL server? Some policy settings maybe? Since Computer #1 was designated as a server to the old POS system, could the software publishers have configured it with tighter security that is blocking the outbound connection from unknown 3rd party apps, but not from SQL Server Management Studio?

Is there maybe a network utility that can monitor a specific app's network connection attempts and identify where/why the connection is being blocked?

I'm not an expert at this stuff so if you reply, please be specific about where and how to fix the problem. Or at least some tips on how to diagnose it.

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If you don't know what components the other POS software might have installed, just reinstall Windows on the machine. This is likely a much faster process than digging around a system you don't know. – Sven Mar 31 '11 at 23:07
It's tempting, unfortunately the old POS software must remain in use until the moment the new software is ready to take over. Can't afford any down time. – MindOfSound Apr 3 '11 at 18:20
Can you complete a test connection by setting up an datasource connection under Control Panel? – BigHomie Mar 25 '14 at 14:30

I would posit that the problem is the sofware. If you can connect to the database server via SSMS that should be telling you that everything up to the application is working correctly: hardware, cabling, TCP/IP, DNS, etc., etc.

I would look for a system, user, or file DSN that may be configured incorrectly, an entry in the hosts or lmhosts file, the application options or settings, an ini file or registry settings related to the application, etc., etc.

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